The present invention relates generally to thin provisioning and tier management and, more particularly, to a method and an apparatus to move page between tiers.
In recent years, thin provisioning has become popular. Thin provisioning is a method for allocating area for a storage system that receives a write command to an unallocated area and allocates physical devices in response to the write commands. Storage systems may also reallocate frequently accessed allocated area to fast and expensive media and rarely accessed allocated area to slow and cheap media. For example, as shown in FIG. 2, a page 102 on a virtual volume 101 is mapped to a page 106 on a tier 2 logical volume 105. When the number of accesses to the page 102 increase, the storage controller copies data on the page 106 to a page 104 on a tier 1 logical volume 103 and changes the link between the page 102 and the page 106 to the page 102 and the page 104.
In actual implementation, a high percentage of pages on the virtual volume tend to be allocated to the higher tier volume and a lot of pages allocated to the lower tier volume are moved to the higher tier volume when there are frequent accesses to the page. When the number of accesses to the page 102 decreases and the number of accesses to another page on the lower tier increase, the frequently accessed area on the lower tier should be moved to higher tier and the less frequently accessed area on the higher tier should be moved to lower tier. However, it tends to take a relatively long time to copy the page 104 to the page 106 since the data transfer rate is relatively low for storage devices allocated to lower tier volume.